How can the resonant frequency of a Hertz antenna be lowered?

The correct choice reflects an understanding of how inductance affects the resonant frequency of an antenna. By placing an inductance in series with the antenna, you are essentially altering its electrical characteristics. Inductance has the effect of adding reactive impedance that shifts the resonant frequency downwards.

In an antenna, the resonant frequency is determined by its physical dimensions and the reactive components present. For a Hertz antenna, which typically is a half-wave dipole, its natural resonant frequency is determined by its length. When you introduce inductance, it creates a situation where the antenna behaves like a tuned circuit. The inductive reactance increases at a given frequency, which requires a lower frequency to reach resonance effectively.

Shortening the antenna or increasing its height does not accommodate the lowering of the resonant frequency in a direct and effective manner. A capacitor in parallel would typically resonate at higher frequencies due to the decrease in reactance at that range, not lower it. Therefore, adding inductance is the most effective method to lower the resonant frequency, making it the right choice.